The present invention relates to a power amplifier and to a transmitting or transceiving apparatus including such a power amplifier. More particularly the present invention is concerned with the control of the power output of the amplifier so that for example it can be increased and decreased at a predetermined rate between a minimum level and a required level which is determined to suit a desired transmission range.
The ability to control the output power of a power amplifier in this way is of importance in many control and telecommunications applications. In digital telecommunication system which operate in accordance with a TDD (time division duplex) format, such as CT2, or in accordance with a TDMA (time division multiple access) format, such as GSM (Group Special Mobile) and DECT (Digital European Cordless Telephone) systems, the rate of ramping up to a required level and ramping down is specified in order to reduce the generation of transient sidebands and interference on adjacent channels which may occur with hard switching of the power amplifier.
For convenience of description, the present invention will be described with reference to a power amplifier for use in the GSM (Group Special Mobile) digital cellular telephone system. However the arrangement may be used in other applications requiring the transmission of pulses of r.f. digitised signals. The GSM system is a TDMA one having a signalling format which includes a framed structure commprising eight slots each having a duration of approximately 570.mu.S. Transmission are made in one slot and reception is in a predetermined one of the remaining seven slots. An 8:1 time compression is used which results in a peak to average transmit power level of 8:1. A feature of the GSM specification is that the power amplifying arrangement must be capable of adaptive power control in that the transmitted power is adjusted in steps to suit the required transmission range. The size of the steps is 2 dB and there are 16 steps ranging from a maximum peak power of 43 dBm to a minimum peak power of 13 dBm. Another factor which has to be taken into account, particularly because the GSM system is a TDMA one, is the frequency spectrum specification which is achieved by shaping of the leading and trailing edges to reduce the risk of generating sidebands and interference on adjacent channels. The shaping of these edges may vary from one power step to the next but it is particularly important to apply shaping to the higher powered signals because the power has to be ramped to the required level in a maximum of 28 .mu.S. A typical shaping profile is known as raised cosine. In order to achieve such a profile a control voltage is applied to the power amplifier, which may comprise a module formed by several successive amplifying stages, to control the power during the ramp and the burst. British Patent Specification 2 220 808 A discloses one method of deriving such a control voltage which makes use of a microcontroller to supply a succession of digital values to a digital to analogue converter (DAC) which in turn produces a train of pulses of predetermined amplitudes.
Power amplifiers can be controlled by either a completely open loop system or using negative feedback. In an open loop system a ramping pulse generator is coupled to a control input of the power amplifier. The relationship between the output power and the control voltage is determined by making measurements of a sample batch of power amplifiers and setting the control voltage to give the required output power with suitable ramps.
In a closed loop system a feedback voltage related to the output power is used to control the output power. Such closed loop systems are known per se from British Patent Specification 2 220 808 A.
A simplified block schematic diagram of a transceiver including a power amplifier having a closed loop control system is shown in FIG. 1 of the accompanying drawings. A power amplifier 10 has a signal input 11 for a signal from a modulator 12 and an output 14 for an amplified signal to be supplied to an antenna 15 by way of a duplexer 17. A portion of the amplified signal is coupled out using a quarter wave coupler 16 and is supplied to a detector system 18. An output of the system 18 is supplied by way of a resistor 20 to the inverting input of a comparator 22 whose output is connected to a control input 24 of the power amplifier 10. A capacitor 26 is coupled between the output of the comparator 22 and its inverting input. The capacitor 26 and the resistor 20 form a filter. A reference signal is applied to a non-inverting input of the comparator 22. The reference signal, which is indicative of the raised cosine ramps and the required power burst, is derived from a reference signal block 28 which is constituted by a microcontroller 30 which feeds a succession of predetermined digital values to a DAC 32 which in turn produces a succession of pulses which undergo shaping in a pulse shaper 34 before being applied to the non-inverting input of the comparator 22. A receiver 19 is coupled to the antenna 15 by way of the duplexer 17.
A drawback of this type of closed loop system is that the detector sensitivity is such that no appreciable output is obtained from the detector system 18 below approximately 0 dBm. However the GSM specification 05.05 states that the power at the start and end of a burst must be -36dBm and that the move to the final required power level must be completed in a maximum time of 28 .mu.S. FIG. 2 illustrates the ideal raised cosine ramp which will not generate undue interference. In reality though a control loop responding at this speed cannot be made to track with the input control voltage on the control input 24 due to the lack of output from the detector system 18 at low power levels. The effect of this lack of control is that there is a rapid initial increase in output power from the power amplifier 10, which initial increase produces an r.f. spectrum outside the GSM specification. This r.f. envelope is illustrated in FIG. 3.